Photolithography has important applications in many areas, including: the printing of graphic-arts, the production of integrated electronic circuits, and the production of flat panel displays, especially active matrix liquid crystal displays (AMLCD).
An ongoing trend in manufacturing integrated electronic circuits is the decrease in structural dimensions and the increase in circuit size and complexity. Current integrated circuit technology offers line widths of less than 1 .mu.m with more than a million transistors squeezed into a single "chip". In laboratories, devices with structure sizes less than 0.5 .mu.m have been realized. In the 1990s, structure sizes smaller than 0.5 .mu.m will be needed, e.g., for the production of 16 Mbit memory chips.
The decrease in the structural dimension of electronic circuits together with the increase in circuit size poses a fundamental problem for current photolithography systems. Similar problems exist for the photolithography systems used to produce high resolution, flat panel displays. It is extremely difficult to simultaneously achieve high resolution and large field-of-view with a lens system. A high resolution lens implies a small depth of focus. This small depth of focus in turn limits the field-of-view in two ways. First, it is difficult and expensive to design a lens with a large flat image plane. Second, and more important, a small depth of focus places tight tolerances on the flatness of the substrate. Producing substrates with small variation over a large field-of view is difficult and expensive. In addition, optical imaging systems with high resolution and large fields-of-view can introduce serious field curvature at the periphery of the image. This makes the overlay of successive exposures problematic since successive exposures are typically performed on different photolithography systems with different degrees of field curvature.
The trade-off between resolution and field-of-view can be improved by using shorter wavelengths of light. However, the optical material available for the deep UV and soft x-ray regions are limited and the associated optical design difficult.
In order to overcome the fundamental limits between resolution and field-of-view, some form of mechanical scanning is necessary. Optical scanning by X-Y translation can achieve high resolution and large fields-of-view; however, this approach can only achieve low data rates because of the low speeds of the translation stage.
Optical steppers are widely used for optical lithography. However, a stepper uses a start-stop action which is inherently slow due to stepping and settling time. In addition, using a stepper to generate a larger pattern requires alignment of adjacent exposures. This requires a careful alignment to stitch together adjacent subpatterns. This stitching process is a source of error (i.e. stitching error) which reduces the effective throughput of the system.
A lithographic system based upon a lens array was recently proposed for the production of flat panel displays. This system uses a two-dimensional lenslet array to image a photomask onto a substrate. Although this approach can achieve a large field of view, it can not achieve high resolution. The lenslet array is a solid unit and cannot account for variations in the flatness of the glass substrate used to produce panel displays. As a result, focus error will limit the resolution of this approach.
In U.S. Pat. No. 4,163,600, issued Aug. 7, 1979 to Russell, U.S. Pat. No. 4,611,881, issued Sept. 16, 1986 to Schmidt et al., and U.S. Pat. No. 5,216,247 issued Jan. 19, 1993 to Wang, et.al., a set of optical scanning techniques were described which use a rotating optical system to scan a point of light across a substrate along a circular arc. These scanning techniques could be used to achieve a direct write photolithography system which provides high resolution, large field-of-view, and relatively high data rates. In addition, this approach can use an autofocus mechanism to account for variations in the substrate flatness. However, to achieve high data rates, the rotating optical system must be spun at very high speeds. The rotation rate can be reduced while maintaining the same writing data rate by simultaneously scanning multiple points; however, this leads to a more complex and expensive system. Furthermore, even with multiple sources, the rotation rate of the optical system is still high enough to pose serious technical challenges due to the high centrifugal force and aero-optic effects.